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As we gaze into our crystal ball...

This is the part everyone is most interested in; the part where we tell you that the comet will light up our bedrooms at night, and make the full moon blush with shame! Except we're not saying that, because that really won’t happen. But don’t lose heart because the reality, albeit uncertain, does still look very promising!

We’ve already said that comets are unpredictable, and this one will be no exception. The closer it gets to us, and the longer we observe it, the better the handle we’ll have on how it will behave. The question we’ve probably received the most is: “When will we know how bright ISON will be?”. We always felt that by August or September of 2013 we should be able to paint a good picture of its expected behavior, and if you read the many blog posts on this site, you should see that we have done that. The comet has continued to perform more-or-less exactly as we were predicting back in February or March, 2013, and so at time of updating this page [Oct 23, 2013] we are comfortable to stand by our earliest predictions. However, comets frequently do unpredictable things, and as ISON is both a Sungrazer and a dynamically new comet, it's future outcome is particularly uncertain. So, that disclaimer aside, let’s take a look at the possible outcomes for Comet ISON and consider three cases that are representative of the spectrum of reasonable outcomes for the comet.

Case #1: Epic Fail

First case is the worst case: Comet ISON is an “epic fail”, to use the popular vernacular. By this we mean that the comet enters into the inner solar system and simply fizzles out, perhaps by way of disruption or fragmentation. Why would it do this? Well, ISON is likely a very “fresh” comet on its first visit to the inner solar system. All of its volatile elements may be fully intact, and it may never have felt the intensity of the Sun’s radiation or gravitational stresses. If true, this could explain why ISON is already so relatively active while still so distant from us. Why is that bad news? Like a light bulb that shines a little too brightly, it may be releasing its ice, dust and gases at a rate that it can not sustain and could lead to it becoming structurally unstable, or simply vaporizing away. The ridiculously over-hyped Comet Elenin did just that, and that is just one recent example of which we have several. If ISON does fizzle, it could happen at any time later this year, whether it be months, weeks, or days from its perihelion, and under this circumstance would be unlikely to reach a brightness that would allow it to attain naked-eye visibility.

Case #2: Sizzle and Burn

Second possible case: we get a nice bright comet, that completely disrupts and vaporizes near the Sun. This is a middle-of-the-road outcome, and one that sees comet ISON brighten up through October and November, coming to perihelion in late-November as a bright comet, quite possibly creeping into negative magnitudes in mid-to-late November. Observers will be able to see it with telescopes, particularly throughout October, before it begins to get lost in the Sun’s glare later in October. In mid-November it will appear in our telescopes on the NASA STEREO and ESA/NASA SOHO satellites, likely saturating our detectors (which struggle with objects brighter than about magnitude zero). The comet peaks in brightness in the hours surrounding its perihelion passage but this tough little newcomer to the solar system begins to struggle as the intense solar radiation, and tidal gravitational stresses begin to take their toll, and in the hours following perihelion, the comet begins to dissipate. It may “outburst”, creating a large dusty plume (a la Comet Holmes), but lose its central nucleus. It may become visible as a diffuse object in the night sky, perhaps visible to the naked eye, but fading fairly quickly over the following weeks, and largely gone from all but the most powerful telescopes by early 2014.

Comet Ikeya-Seki, visible in the dawn sky

Comet Lovejoy in 2011 gives us a hint of how Comet ISON might appear in SOHO in the second case, saturating the detector and sporting a long thin tail.

Case #3: History in the Making

Third case: “the Comet of the Century”. We're almost hesitant to even describe this outcome because folks will jump all over this. Nonetheless, the possibility does indeed exist that Comet ISON becomes one of the more spectacular comets in recent memory. In this circumstance, the comet will brighten steadily and quickly through September, becoming easily visible with binoculars and perhaps even naked eye in some locations by early-to-mid October, particularly for Northern Hemisphere observers. It is gradually lost in the Sun’s glare in late October, but remains bright enough that skilled observers can follow it with their telescopes until a week or so preceding perihelion. In late November, it enters our spacecraft imagers, overwhelming the detectors and pushing us to the limits of our shortest exposure times to avoid saturation. In the day-or-so before perihelion, it brightens by orders of magnitude as the sunlight illuminates its long dusty tail. It may briefly peak near magnitude -8 or -10, not brighter than a full moon (and certainly NOT bigger than the full moon!), but is visible in broad daylight to careful observers who block the sunlight with their hand or the edge of a building. After perihelion, it emerges from the Sun’s glare as a large, bright comet, maybe still magnitude -3 or -4. At this time it should be sporting a very long dusty tail that stretches far across the night sky, quite likely reminiscent of that of its (alleged) analog, C/1680 V1, or the great Kreutz Sungrazer Ikeya-Seki . Northern hemisphere observers will have the best view, but even those in the Southern Hemisphere will get to enjoy the spectacle for several weeks, into 2014, as the comet slowly recedes away.

In conclusion...

We hate to belabor a point but these are theorized outcomes that represent parts of an entire spectrum of possible outcomes. The actual behavior of Comet ISON could fall anywhere within this regime, and have one or more facets of the above. That is, fragmentation/vaporization could happen at any point or not at all, and would be completely irrespective of its brightness at the time. Comets are notoriously unpredictable, and this one is one of the least predictable in a very long time. But regardless of the outcome, the build-up, anticipation and excitement already makes it more than worthwhile!
OCTOBER UPDATE: This page was created in early 2013, when we really had very little information to go on. At time of writing this update - October 23, 2013 - we now know a little more about the comet. With regards to the above scenarios, none of them are out of the question yet. It could still fade out at any point, though given the tremendous number of observations and science data we have already recorded for the comet, I think to call it "an epic fail" would be a particularly harsh label. Likewise, it is extremely unlikely to be "the Comet of the Century", and we have never felt that likely anyway (we didn't coin that phrase: that was exclusively the media!). ISON has continued to brighten gradually, and pretty much exactly as we expect from an Oort Cloud comet. We see no signs of "fizzling", and no signs that it will be one of the most spectacular comets in living memory (though the latter is subjective. Instead we think it will continue its trend of gradually brightening up, with a sharp increase around perihelion that perhaps takes it to -3 to -5 magnitude for several hours on Novermber 28, 2013. The big question that does remain - and indeed will remain unanswered until after perihelion - is whether it will disrupt and vaporize at perihelion. We lean towards thinking it will survive, but neither outcome would be an enormous surprise to us.
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Image of the Week

Every week, a new image related to Comet ISON!

This week: The Final Nail in the Coffin? When ground-based observers came up empty-handed in the hunt for ISON's remains, we turned to the Hubble Telescope to see if its ultra-powerful instruments could find anything...Earlier picks can be found in the ISON Image of the Week Archives. Note that this feature will become monthly, instead of weekly, in 2014.